Eight19 Limited Advanced Electron Transport Materials for Application in rganic Photovoltaics (PV) Alan Sellinger GCEP Research Symposium 2011 ctober 5, 2011 1
Why Solar? 1 % of land with solar cells could meet our electricity needs 6 Boxes at 3.3 TW Each Amount of solar energy in hours, received each day on an optimally tilted surface during the worst month of the year.
Total PV installation: Germany 50% USA 9%! 3
Types of Solar Technologies Lowest cost $/W 4
Why rganic Semiconductors? Attractive properties: Abundant: ~100,000 tons/year Mature industry/markets Low materials cost: ~$1/g 15 /m 2 Low-cost processing: printing Excellent stability on-toxic CuPc Copper Phthalocyanine 5
rganic LEDs for Displays
rganic LEDs for Lighting
ew Applications for rganic Solar Cells
Chemistry obel Prize Day! rganic Electronics: Combination of 3 obel Prizes! 1996 obel Prize in Chemistry - Robert F. Curl Jr., Sir Harold Kroto, Richard E. Smalley for their discovery of fullerenes (C60) C60 C70 2000 obel Prize in Chemistry - Alan Heeger, Alan G. MacDiarmid, Hideki Shirakawa for their discovery and development of conductive polymers S n PEDT 2010 obel Prize in Chemistry - Richard F. Heck, Ei-ichi egishi, Akira Suzuki for palladium-catalyzed cross couplings in organic synthesis C-C bond formation S S C 6 H 13 S C 6 H 13 S C 6 H 13 S n/4 C 6 H 13 9
Motivation for rganic-based PV Lower production cost Roll-to-roll coating Inexpensive active materials Inexpensive processing Lower capital cost Lower environmental impact - $/W Inhabitat.com Energy Payback Time (yrs) C 2 Emission in Production (g) Crystalline silicon 2.3 1560 Thin film (CIGS, CdTe, etc.) 1.2-3.2 900-2560 Polymer PV (Glass substrate) 1.3 820 Polymer PV (Plastic substrate) 0.2 130 Roes et al., Prog. Photovoltaics. 2009 (17) 372-393 10
Efficiency Cost Analysis for Solar Cell Technologies Thin Film Crystalline Si rganic/hybrid near term/today today future $/W basis competitive Paint Material and Manufacturing Cost Ingot
Anode What is an rganic Photovoltaic? Simplified Working Principle Recombination Photon + - Exciton + - Hole Charge Separation + + Electron + + - - - - Cathode Photon + - Generation + - + - + - Diffusion Drift/Diffusion Donor Material 50-200 nm Acceptor Material Kietzke, T., Adv. ptoelect., 2007, Article ID 40285.
Progress in rganic Solar Cells 10%? Mitsubishi 2011 Solution processes (Polymer, PCBM) Vacuum processed 9% Solarmer 2011? 8.3% Heliatek 2010 2005 2010 Similar development for organic solar cells as for amorphous silicon 20 years ago Both approaches are reaching nearly similar efficiencies Improvements are largely materials discovery based
Estimated lifetimes of PVs Burn-in loss Burn-in time Lifetime in linear regime* P3HT:PCBM 16% 55 days 3.5 years PCDTBT:PC70BM 27% 38 days 6.7 years *Lifetime assumes 5.5 hrs/day of one-sun intensity 1 Heliatek (BASF and Bosch) has achieved 30 year lifetimes with tandem PVs 2 1. C. Peters, M. McGehee, Adv. Energy Mater. in press, 2011 2. www.heliatek.com
Bulk heterojunction (BHJ) Polymer Solar Cell Advantages: Solution processing (roll-to-roll), increased contact area between active phases, Disadvantages: Current world record: 8-10% Extrapolated lifetimes >6 years Difficult to achieve optimum morphology, polymers generally less pure than molecules, solvent dependence http://www.solarmer.com/
High-efficiency BHJ solar cells PBT7:PC 70 BM 7.4% PCE PCDTBT:PC 70 BM 6.1% PCE PC70BM PC 70 BM Liang et al., Adv. Mat. 2010 (22) E135-E138 Park et al., at. Phot. 2009 (3) 297-303 P3HT:ICBA* 6.5% PCE * ICBA: Indene-C60 Bisadduct PC 60 BM Zhao et al., Adv. Mat. 2010 (22) 4355-4358 Record PV cells ALL use fullerene derivatives Recent development based on donor materials discovery 16
Absorption Coefficient (M -1 cm -1 ) Drawbacks for fullerenes 2.5 x 105 2 1.5 ew Acceptor PC 60 BM PC 70 BM -3.7 ev -4.3 ev -3.1 ev P3HT -3.4 ev -3.9 ev -3.1 ev P3HT 1 0.5 0 300 400 500 600 700 800 Wavelength (nm) -5.6 ev -6.1 ev PCBM -5.1 ev -5.9 ev -6.1 ev ew Acceptor -5.1 ev Reduced solar spectrum absorption Lower V C Difficult synthesis and purification = Higher cost PC 60 BM: $50/g CuPc: $1-5/g Fullerene acceptor up to 10% of total PV system cost Roes et al., Prog. Photovoltaics. 2009 (17) 372-393 17
Effect of Production Steps on Cost PC60BM PC70BM Significant increases (2X) for higher purification PC 71 BM approximately 3.4X more expensive than PC 61 BM 18 Anctil et al., Environ. Sci. Technol. 45, (2011) 2353-9
on-fullerene acceptors P3HT:99 BF ~2% PCE C PPT:C-PPV 2% PCE C Holcombe et al., J. Am. Chem. Soc. 2009 131 (40) 14160-14161 F F F F F B Cl Cl F F F F F F Verreet et al., Adv. Ener. Mat.. 2011 (1) 565-568 n SubPc:FSubPc dimer 2.5% PCE (vacuum) F B F F F F C C Brunetti et al., Ang. Chem. Int. Ed. 2010 49 (3) 532-536 S Woo et al., Chem. Mater. 2010 22 (5) 1673-1679 C C PPT:EV-BT 1.4% PCE Lower efficiencies due to lower J SC and lower FF 19
ew Acceptors based on Vinazene riginal applications as high nitrogen containing materials for reduced flammability. Without vinyl groups has been used in the agriculture sector (fertilizers) Their highly electron deficient properties make them candidates as acceptor materials in organic electronic applications R R H H H C C C Vinazene 2-vinyl-4,5-dicyanoimidazole C C C 4,5-dicyanoimidazole Commercially available
Alkylation of 2-Vinyl-4,5-dicyanoimidazole Acetone, K 2 C 3 Reflux I C C 1-Butylvinazene (90%) H Acetone, K 2 C 3 I C Vinazene C Reflux C C 1-Hexylvinazene (90%) Br DMF, K 2 C 3, 70 o C C C 1-Ethylhexylvinazene (56%)
Tunable Synthesis: Heck Chemistry Shin R.Y.C., Sonar, P., Siew, P.S., Chen, Z.C., Sellinger, A., J. rg. Chem., 2009, 74 (9), 3293 3298.
ormalised Intensity (a.u.) ormalised Intensity (a.u.) ptical properties of selected vinazene derivatives S S S (1) (4) S (5) (10) (13) 1.0 0.8 1 4 5 10 13 1.0 0.8 1 4 5 10 13 0.6 0.6 0.4 0.4 0.2 0.2 0.0 300 350 400 450 500 550 600 650 Wavelength (nm) UV spectra of the molecules in toluene 0.0 350 400 450 500 550 600 650 700 750 Wavelength (nm) PL spectra of the molecules in toluene Significant absorption in visible spectrum
Vinazenes used for PVs Solubilizing groups for solution processing Conjugated chemical links S HV-BT C C C C Electron accepting sites EV-BT Better solubility Can be thermally sublimed as well
Preparation of PV Devices PPT donor polymer ew Electron Acceptor Solution processed active layer 25
Kietzke, T., Shin R.Y.C., Egbe, D.A.M., Chen, Z.K., and Sellinger, A., Macromolecules, 2007, 40, 4424-4428. Shin R.Y.C., Kietzke, T., Sudhakar, S., Dodabalapur A., Chen, Z.K., and Sellinger, A., Chem. Mater., 2007, 19(8), 1892-1894. Woo, C.W., Holcombe, T.W., Tam, T.L., Sellinger, A., Frechet, J.M.J., Chem. Mater. 2010, 22 (5), 1673 1679 PPT with HV-BT PPT Voc = 0.62 V, FF = 0.40, PCE = 1.41%
Can we make better acceptors? Solubilizing groups for solution processing Conjugated chemical links S C C C C Electron accepting sites Corresponding electron donor has charge mobility (cm 2 /V sec) in 10-4 range. Is charge transport mis-match a problem?
Improving rganic Acceptor Materials
Improved Acceptor Materials? S
Acceptor Materials: Computational Studies Ground-state geometries LUM HM
Synthetic Scheme for PI-BT/I-BT PI-BT PI-BT I-BT I-BT 31
ormalized Absorption (arb. units.) ew Acceptor Properties -3.1 ev -3.1 ev 1 0.8 Thin Film PI-BT I-BT -3.69 ev P3HT -3.84 ev P3HT 0.6 PI-BT -5.1 ev I-BT -5.1 ev 0.4 0.2-6.05 ev -5.99 ev 0 200 300 400 500 600 700 800 900 Wavelength (nm) E LUM ~ 0.6 ev E HM ~ 0.9 ev E LUM ~ 0.7 ev E HM ~ 0.9 ev Peak acceptor absorption is in visible spectrum Larger E LUM,Acc E HM,Don than for P3HT:PC 60 BM Tunable LUM expected higher V oc than fullerenes 32
Current Density (ma cm -2 ) J-V Curve ptimal Devices 8 6 4 2 0-2 -4-6 PI-BT I-BT -8-1 -0.5 0 0.5 1 1.5 Voltage (V) P3HT:PI-BT P3HT:I-BT Solvent Chlorobenzene Chloroform Thickness (nm) ~ 90 ~ 120 Cathode LiF(1nm)/Al Ca(7nm)/Al Anneal Pre/Post Post Pre Ann. Temp/Time 110 C/3min 110 C/10min Jsc (ma/cm 2 ) 4.7 1.2 Voc (V) 0.96 0.51 FF 0.56 0.35 PCE (%) 2.54 0.22 Believed highest efficiency BHJ with non-fullerene acceptor and commercially available P3HT donor High voltage as expected with higher-lying LUM Why is the efficiency for I-BT 10X lower? 33
EQE (%) or Absorption (arb. units) EQE Spectra of P3HT:PI-BT Device 0.5 0.4 P3HT:PI-BT EQE P3HT Absorption PI-BT Absorption 0.3 0.2 0.1 0 300 400 500 600 700 800 900 Wavelength (nm) Significant photocurrent generation from acceptor absorption 34
Molecular Simulation Ground State Geometry 27.3 2.06 2.06 PI-BT I-BT Twisting of I-BT molecule due to steric interactions may prevent crystallization in films 35
GIXS Acceptor Crystallization P3HT:PI-BT (CB) P3HT:I-BT (CF) P3HT nly - CB P3HT nly - CF More acceptor peaks in PI-BT sample than I-BT 36
PV Conclusions Very promising new acceptor materials as potential replacement for fullerene derivatives Synthesized in minimal step/moderate yield reactions Tunable energy levels Anticipated to be much less expensive than fullerenes Initial device performance very promising From initial PCE of 0.1% >2.54% PCE max, 5 years of R&D, (5-7 total researchers) Close interaction between chemists and device engineers/physicists
Acknowledgements Co-PI - Prof. Michael McGehee Synthesis Dr. Xu Han (now at DuPont R&D Shanghai), Dr. Andrew Higgs Device Fabrication/Characterization Jason Bloking, Jack Kastrop Quantum Calculations Dr. Chad Risko, Dr. Joe orton, Laxman Pandey, Dr. Jean-Luc Bredas (Georgia Tech) Funding Sources: 38